Well, my money is still on the freewheel being the culprit. The fact that the problem is so intermittent, and that you've checked everything else out so thoroughly, all seem to point to the freewheel, IMO. The derailleur and jockey pulleys are so simple--just like you said--that it's hard to imagine what they could possibly be doing that would cause that symptom of pedaling fast but the wheel not being driven. If the jockey pulleys were extremely worn, that might cause some sloppy shifting, but the chain still would be either in one gear or another. Same with the derailleur. Maladjustment would cause chain noise or slop in the shifting, but I can't think of how it would cause the current issue. If it were the front derailleur dropping the chain, she would hear that and would notice it, so it can't be that.
I would probably take the rear wheel off and spin the freewheel, listening carefully to the sound of the pawls and also for the sound of grit. It's possible to have grit, or even corrosion, inside the freewheel that can keep the pawls from engaging. If there's any chance that it could have considerable grit in it, or excess oil or grease, or if the old grease in it has degraded and stiffened, then it should be removed from the hub, and flushed really well in solvent (you won't believe the stuff that comes out of them), then relubricated with a nice light oil. Tri-Flow is great. I used to use Phil Tenacious Oil, but even that can be too thick. These things are only moving inside when there's no load on them, so they don't need thick oil in there.
Is there any chance that someone else put something bad in there, like 3-in-1 oil, or anything else? Sounds like you're the only one doing maintenance, but thought I'd ask.